1. Technical Field
The invention is related to radio frequency (RF) plasma reactors for processing semiconductor wafers.
2. Background of the Invention
A conventional parallel plate plasma reactor of the type illustrated in the highly simplified diagram of FIG. 1 includes a vacuum chamber 10 having a side wall 12 and top and bottom parallel insulated capacitor electrodes 14, 16. The side wall 12 may be grounded and is isolated from both electrodes 14, 16 if they are both RF-powered. Alternatively, if one of the electrodes 14, 16 is grounded then it is also connected to the side wall 12 while the other electrode is RF-powered. Typically, one of the electrodes 14, 16 is connected through an RF impedance match network 18 to an RF power generator 20 while the other electrode is grounded and connected to the side wall 12. A semiconductor wafer 22 is placed in the chamber 10 and a processing gas is introduced into the chamber. The RF power applied across the pair of electrodes 14, 16 ignites a plasma in the chamber 10 and maintains the plasma by capacitive coupling.
The parallel plate plasma reactor of FIG. 1 enjoys several distinct advantages. Specifically, the large area and closeness of the parallel electrodes 14, 16 readily ignite the plasma and permit precise control of the capacitive coupling and plasma ion energy at the wafer 22. The plasma processing of such a reactor is highly reproducible in comparison with other types of plasma reactors. One disadvantage of the parallel plate plasma reactor is that RF energy is coupled to the plasma through capacitive coupling almost exclusively, so that plasma ion density can be increased or decreased only by increasing or decreasing capacitively coupled RF power. However, such a power increase increases ion bombardment energy, resulting in greater device damage on the wafer. Thus, plasma density in such a reactor is limited by the plasma ion energy threshold at which device damage becomes significant.
An inductively coupled plasma reactor of the type illustrated in FIG. 2 has a vacuum chamber 30 surrounded by a coil inductor 32. A semiconductor wafer 34 is supported inside the chamber 30 on a wafer pedestal 36. The coil inductor 32 is connected to an RF power generator 38 through an RF impedance match network 40. A processing gas is introduced into the interior of the chamber 30 and RF power inductively coupled from the coil 32 ignites and maintains a plasma within the chamber 30. Plasma ion density is readily increased by increasing the RF power applied to the coil inductor 32. However, such an increase in applied RF power on the inductor 32 does not concomitantly increase the energy of ions impinging on the wafer surface. Thus, plasma density can be increased to a much greater extent than in the reactor of FIG. 1 without risking device damage due to excessive plasma ion energy, a significant advantage.
One problem with the inductively coupled plasma reactor of FIG. 2 is that the inductor coil 32 provides little or no control of plasma ion energy, and therefore a separate apparatus for providing such control must be provided. This problem is conventionally solved by applying RF power to the wafer pedestal 36 from a capacitively coupled independent RF power generator 42 through an RF impedance match network 44. The RF power generator 42, controlling plasma ion energy, may be adjusted independently of the RF power generator 38 controlling plasma ion density, so that plasma ion energy control and plasma ion density control are decoupled.
However, the bias RF power applied to the wafer pedestal 36 provides inferior control over plasma ion energy and provides inferior process reproducibility (in comparison with the parallel plate plasma reactor of FIG. 1), a significant problem. Another problem is that the inductively coupled reactor of FIG. 2 does not ignite a plasma as readily as the capacitively coupled reactor of FIG. 1.
It has not seemed possible to realize the disparate advantages of both the capacitively coupled plasma reactor of FIG. 1 and the inductively coupled plasma reactor of FIG. 2 in the same reactor, due to the seemingly conflicting structural features required for realizing the different advantages.
It is an object of the present invention to provide all of the advantages of both the capacitively coupled reactor of FIG. 1 and the inductively coupled reactor of FIG. 2 in a single plasma reactor in a reactor.
It is a further object of the present invention to employ RF power-splitting to deliver both inductively coupled and capacitive coupled RF power into the process chamber with a minimum number of RF generators and in a well-controlled fashion.
It is another object of the present invention to provide all of the advantages of both the capacitively coupled reactor of FIG. 1 and the inductively coupled reactor of FIG. 2 in a single plasma reactor in a reactor in which contaminant deposits on the interior surface of the ceiling are sputtered away to keep the ceiling clean.
It is yet another object of the present invention to provide a way of sputtering material from the chamber ceiling to supply processing species into the chamber.